Integrated circuit chips such as application specific integrated circuits, (ASICs) and very large scale integrated circuits, (VLSIs), are manufactured with a limited number of connection pins which allow communication to and from the environment external to the chip. For example, an input pin permits a chip to receive data from the surrounding board logic while an output pin permits the chip to send data outward to the external board logic.
Due to the size of integrated circuit chips, it is unfortunate that the number of pins available for input-output communications are limited due to the small size of the integrated circuit chip, and it is most important to optimize the use of the input pins and the output pins.
In any chip there are signals that are absolutely necessary for proper operation of the chip and require dedicated input and output pins in order to perform the functions that the chip logic and the board logic require. However, with the ability to provide other signals in addition to the absolute necessary signals, it will be found to be a valuable asset if certain other enhancement signals can be used and assigned to the pins as well. This enhancement could be the situation for two possible reasons:
(1) The added visibility of signals or state conditions inside the chip can be useful in a debugging environment;
(2) In situations where problems are encountered, additional input signals, if available, could help control some of the logic inside the chip, while some enhancing additional output signals could also help control some of the external board logic, especially in programmable array logic, PALs.
With the added flexibility as indicated by (1) and (2) described above, this flexibility can prevent the need to redesign a chip which would be costly in time, in scheduling and money involved.
It may be observed that there could be a more costly alternative by the use of a much larger chip with a greater quantity of available pins. However, in terms of both price of the chip and the required extra space that would be required in mounting on a PC Board, it would be seen that this is not a very desirable option. Thus it is most cost effective and important to make the optimal use of all the pins that are available on a chip.
One method of providing a greater optimal use of the available pins that come with an integrated circuit chip would be to provide circuitry and methods by which certain of the fixed pins in the chip could be used in a dual capacity, that is to say sometimes used as a pin for input signals and sometimes used as a pin for output signals depending on certain controlled conditions.
The object of the presently described system and method is to make it possible for certain ones of the scarce connection pins on an integrated circuit chip to be used in a dual functional capacity, that is, sometimes to be used for bringing input signals in and sometimes to use the same pin for output signals out to external units.